The cornea is the outermost fibrous transparent coating of the eye which resembles a watch glass. The curvature of the cornea is normally somewhat greater than the rest of the globe of the eye and is ideally spherical in nature. The central third of the cornea is referred to as the optical zone or visual pathway with a slight flattening taking place outwardly thereof as the cornea thickens towards its periphery. Most of the refraction of the eye takes place through the cornea.
There are a number of dystrophies which involve a gross deformation of the cornea and which cause severe vision problems. If allowed to progress, such dystrophies can lead to blindness. One of these dystrophies is Keratoconus, in which a bulge or cone develops on the cornea. For a cornea with Keratoconus, the corneal tissue is thin, and one's normal eye pressure pushing on the weakened thin tissue causes the bulge or cone. This results in complex reflections and scattering patterns of light entering the eye, which prevent the appropriate focusing of an image onto the retina, and the structure of the cornea is significantly weakened. This phenomenon is shown in FIG. 1. In contrast, FIG. 2 shows how light rays pass through the cornea in a healthy, normal eye, providing clear and focused vision. Severe forms of Keratoconus require the implantation of intrastromal or intracorneal ring segments or, in some instances, a corneal transplant. Less severe cases of Keratoconus have been largely addressed with custom-made hard contact lenses.
The cone in a Keratoconic eye manifests differently in each patient. In other words, the cone can vary by position both centrally, as opposed to radially, in addition to the angle as would be observed in a polar coordinate axis. The cone can also vary by size in the sense of the height of the protrusion, known as sagital depth, the cone's slope measured by a “K” or Keratometry reading, and the surface area of the footprint of the cone. In addition, the thickness of the cornea, although different for each individual, typically falls within a normal range. However, for a keratoconic cornea, the cornea is abnormally thin and contributes to its irregular shape in a patient-specific way. The thinning of the cornea affects the cornea's structural integrity, leads to the creation of a bulge or cone and the extenuating visual problems that result.
There are a number of other conditions which are similar to Keratoconus in nature. These conditions include ectasia (corneal thinning), post-LASIK ectasia, pellucid marginal degeneration and other corneal dystrophies. All of these dystrophies lead to gross deformities of the cornea with consequences similar to Keratoconus. Therefore, while the present application discusses Keratoconus in particular, it should be understood that the principles, products and procedures discussed herein are also applicable to other similar conditions.
One method for correcting these disorders is through the implantation of polymeric rings (intrastromal corneal rings) in the eye's corneal stroma to flatten, strengthen and change the curvature of the cornea. Previous work has involved the implantation of polymethylmethacrylate (PMMA) rings, allograft corneal tissue, and hydrogels for common refractive procedures: myopia, hyperopia and astigmatism. One of the ring devices that has been conventionally used involves a split ring design which is inserted into a channel that has been previously dissected in the stromal layer of the cornea. Such a ring is discussed in U.S. Pat. No. 5,824,086, in the name of Silvestrini. Other ring devices use PMMA intrastromal rings which completely encircle the cornea. Such ring designs are disclosed in U.S. Pat. Nos. 4,452,235 and 4,671,276, both in the name of Reynolds, and U.S. Pat. No. 4,961,744, in the name of Kilmer. In each of these rings, a minimally invasive incision is used both for producing a channel and for inserting the implant as a permanent refractive procedure for myopia, hyperopia and astigmatism. However, these rings are not designed for correction of various visual aberrations caused by Keratoconus or other conditions of the cornea, and these designs are not directed to the introduction of therapeutic or diagnostic materials into the corneal intrastromal space.
For corneas with gross deformities or abnormalities caused by corneal disease, there has been a need for an improved product and method for treating these abnormal keratoconic corneas, improve a patient's vision and avoid a treatment of last resort, i.e., a corneal transplant. In these more extreme cases, conventional intracorneal ring segments can normalize the shape of the cornea from within, add structural integrity, flatten the protruding cone and, in the process, eliminate many of the visual distortions and aberrations caused by Keratoconus and other corneal dystrophies, while also stabilizing the progression of Keratoconus.
Although conventional intracorneal ring segments can provide the benefits discussed above, they suffer from a number of technological deficiencies. For example, conventional intracorneal ring segments do not cause a sufficient amount of flattening or reshaping of the cornea when placed outside of the visual pathway or optical zone where light enters the eye. Although it has been determined that a more central placement of the intracorneal ring segments leads to greater flattening efficacy, all known ring segments generate halos, glare and unusual light scattering when placed near or inside the visual pathway, as light rays are defracted by the intracorneal rings themselves across the visual pathway of the eye. Each of these deficiencies make conventionally known and commercially available intracorneal ring segments inadequate in treating the visual problems caused by Keratoconus and other similar corneal dystrophies.